The present invention relates to an oil pump for an automatic transmission, which oil pump is provided with an air vent structure.
In a motor vehicle provided with a torque converter and a typical automatic transmission, oil is supplied by an oil pump driven by a sleeve extending from an impeller fixed to an output shaft of the transmission. This oil pump sucks up oil from an oil pan located below a transmission case, through a suction passage provided with a strainer, and discharges oil to lubricated portions and control valves of the transmission through a discharge outlet and a discharge passage.
In a hybrid drive vehicle, the transmission may be provided with the oil pump as a main pump, and an auxiliary pump for supplying a sufficient amount of oil to the lubricated portions while the main pump is not driven by an engine. The auxiliary pump is connected to a suction passage and a discharge passage which are arranged in parallel with the suction passage and discharge passage of the main pump.
FIG. 7 shows a reference example in which a suction passage is provided with a strainer 107 at its tip, and strainer 107 has an opening 108 located below a liquid level of oil F100 in an oil pan 103. In the reference example, situations are possible in which when a vehicle is decelerating rapidly, oil F100 in oil pan 103 moves in a vehicle travel direction so that liquid level W100 is inclined with respect to oil pan 103, the opening 108 of strainer 107 is exposed above liquid level 100, and strainer 107 sucks air as shown by an arrow. The suction of air can lower the pump efficiency, and adversely affect normal operation of control valves because oil discharged through a discharge outlet is mixed with air.
Japanese Patent Application Publication No. 2003-239869 discloses an oil pump intended for solving the to problem described above. In this oil pump, first and second grooves are formed between a drive shaft and a bearing. The first groove extends in the axial direction and communicates at one end with a discharge port. The second groove extends in the axial direction and communicates at one end with the outside of the oil pump. The first and second grooves are separated and arranged to partly overlap with each other in the axial direction, so that air escapes from the inside of the oil pump to the outside through the first and second grooves and an intermediate clearance at the overlap section between the drive shaft and bearing.